In a normal four stroke pull-start engine, a starting event moves the engine through one or more engine cycles to start the engine. The starting event may involve a person pulling a pull cord, or an electric starter, rotating the engine. The engine cycle has four strokes: the intake stroke, the compression stroke, the expansion stroke, and the exhaust stroke.
During normal engine operation, an air/fuel mixture is ignited just before the expansion stroke to power the engine and move the engine through the engine cycle. During pull starting, the operator must exert enough force to overcome the resistive force of the compressed air in the combustion chamber during the combustion stroke. The additional force required to compress the air increases the torque on the cord and makes the engine more difficult to start.
A compression release mechanism may be used to release pressure in the combustion chamber during the compression stroke, which reduces the torque and resistive force on the cord. The reduced torque makes the engine easier to start because the operator does not have to exert as large of a force on the pull cord to move the engine through the cycle. Typically, a compression release mechanism slightly unseats an engine valve to vent the combustion chamber during the compression stroke while the engine is rotating at starting speeds. The compression release mechanism generally disengages at or before the engine reaches normal operating speeds.
The object of the compression release mechanism is to reduce the torque on the cord by releasing the pressure in the combustion chamber during the compression stroke. Since the combustion chamber is relatively airtight when the engine valves are closed, the release of pressure during the compression stroke creates a partial vacuum in the combustion chamber for the expansion stroke. When starting an engine having a compression release mechanism, the operator must exert enough force on the pull cord during the expansion stroke to pull the piston against the partial vacuum in the combustion chamber. The additional force required to overcome the partial vacuum during the expansion stroke creates a torque and the resistive force on the cord, and makes the engine more difficult to start.
In some prior art engines, the compression release mechanism may be pivotally retained to the cam shaft with a pin. An aperture extends through the cam shaft, and the pin extends through the aperture. The pin is connected to the compression release member, and retains the compression release member to the cam shaft. The compression release member may pivot about a pivot axis which extends through the pin. Both the pin, and the pivot axis of the some prior art compression release members intersect with the cam shaft.
Creating an aperture through a cam shaft is generally a difficult machining operation. The cam shaft is generally made from a relatively hard and strong material that may be difficult to machine. A harder material is generally more difficult to machine than a softer material. Additionally, the cam shaft is generally round and cylindrical, and machining into a round surface to create an aperture may also be difficult. The round surface tends to deflect a drill tip, and increase the difficulty of maintaining the center line of a drilled hole through the cam shaft. Machining an aperture into a round surface of a hard material creates many manufacturing problems, such as scrapped parts and excessive tool wear.